1. Field of the Invention
The present invention relates to electronic musical instruments, and more particularly to electronic organs and the like in which the musical tones for a plurality of separate octaves are synthesized by frequency synthesizers.
2. The Prior Art
Electronic musical instruments, such as electronic pianos and organs, typically employ a unit called a "top octave frequency synthesizer" for deriving twelve output signals having frequencies corresponding to the musical tones of the highest octave of the instrument by dividing the output of a master clock source by predetermined dividing factors. Signals having frequencies corresponding to the musical tones of the second highest octave are derived from the outputs of the top octave synthesizer by dividing the pulse repetition rate of the outputs of the top octave synthesizer by a factor of two to derive a series of output signals having frequencies equal to one-half the frequencies of the output signals of the top octave synthesizer. Signals for the lower octaves are derived in a similar fashion. In this type system, the relationship between corresponding tones of different octaves is exact, and the corresponding tones of each octave are locked into a precise phase relationship with each other. The frequencies of the tones produced by such an instrument are accurate and serve efficiently to produce musical sounds either singly or in combination. However, when two or more corresponding tones from separate octaves are sounded simultaneously, the result is different than that obtainable in sounding corresponding tones from separate octaves by means of a non-electronic musical instrument, such as a pipe organ, for example. When a non-electronic musical instrument is employed, the corresponding tones are not precise harmonics of each other, but differ slightly therefrom, producing a chorus effect.
It is known from U.S. Pat. No. 3,828,109 to modify the just-described system so as to provide a rolling or moving chorus effect to the musical tones produced thereby. The tone generator described therein employs a plurality of frequency synthesizers, one for each octave of musical tones to be generated in the musical instrument, a source of clock pulses having a pulse repetition rate which is a multiple of the highest frequency desired for the musical tones, the frequency synthesizer for the octave having the highest frequencies being connected to the source, and a plurality of frequency dividers connected in cascade relationship for successively dividing the pulse repetition rate of the pulse train by a factor which differs slightly from two, such frequency dividers being connected individually to the inputs of the frequency synthesizers for the lower octaves. As a result, corresponding musical tones of successive octaves are not locked in phase relationship but have frequencies which differ slightly from pure harmonics so as to present a chorus effect when such tones are reproduced simultaneously.
Although the system of U.S. Pat. No. 3,828,109 overcomes the objection of corresponding musical tones of successive octaves being locked in phase, the chorus effect is limited by the fact that temperament errors in any musical interval are the same in each octave, and if the same interval is sounded simultaneously in different octaves the effect is not the same as that achieved by acoustic instruments in which temperament errors are truly random. In the equitempered scale, each note has a frequency equal to the next lowest frequency multiplied by the twelfth root of two, or 1.0594, an irrational number; accordingly, each note has a precise frequency. However, because of the fact that the frequency-dividing factors of frequency synthesizers are integers, the resultant tones obtained from the frequency synthesizer differ from the corresponding frequencies of the equitempered scale, the amount of the error depending upon the selected frequency of the master oscillator and the dividing factors of the frequency synthesizer used. The error may be in one direction for some of the notes in an octave, and in the opposite direction for others, and the extent of the error may change from note to note. For example, if it is desired to obtain the tones of the twelve notes in the highest octave using the National Semiconductor MM5832/MM5833 frequency synthesizer, and if the frequency of the master frequency oscillator is selected at 2.00024 MHz, twelve tones having frequencies as shown in Table 1 will be obtained. The table also shows the frequencies of the tones in the equitempered scale, the dividing factors of the MM5832/MM5833 frequency synthesizer, and the error (in cents) between the derived frequencies and the frequencies of the equitempered scale.
TABLE 1 ______________________________________ Dividing Derived Equitempered Error Notes Factor Frequency (Hz) Scale(Hz) (Cents) ______________________________________ C.sub.7 478 4184.6 4186.0 -0.56 C.music-sharp..sub.7 451 4435.1 4434.9 +0.07 D.sub.7 426 4695.4 4698.6 -1.14 D.music-sharp..sub.7 402 4975.7 4978.0 -0.77 E.sub.7 379 5277.7 5274.0 +1.18 F.sub.7 358 5587.3 5587.7 -0.12 F.music-sharp..sub.7 338 5917.9 5919.9 -0.57 G.sub.7 319 6270.3 6271.9 -0.43 G.music-sharp..sub.7 301 6645.3 6644.9 +0.10 A.sub.7 284 7043.1 7040.0 +0.74 A.music-sharp..sub.7 268 7463.6 7458.6 +1.13 B.sub.7 253 7906.1 7902.1 +0.85 C.sub.8 239 8369.2 8372.0 -0.56 ______________________________________
It will be apparent from this table that the frequency errors fall within the range from -1.14 to +1.18 cents, or a spread of 2.32 cents. While it would seem desirable to have the smallest termperament error possible, this is not necessarily the case in an electronic instrument. Very pleasant chorus effects arise out of the simultaneous sounding of tones having coincident harmonic partials that are slightly detuned from one another. It is generally true that the more such tones as are simultaneously sounded, the more desirable it is that they have errors, and that the departure from true equitemperament be greater, provided that they are random in sense and magnitude.
U.S. Pat. No. 3,809,787 describes a tone generator including a plurality of master oscillators, each for a different octave and the frequencies of each being different from each other by a ratio of 2:1. Each of the master oscillators is coupled to a respective frequency divider unit, each frequency divider having a different frequency dividing ratio and respectively dividing the frequency of its respective master oscillator down to pitches of different notes in its respective octave. Since the same lettered notes in different octaves are obtained by frequency dividing the signals produced by separate master oscillators, they are independent in frequency and phase and produce a chorus effect when two or more tones from separate octaves or when two keys of the same pitch but in different keyboards are sounded simultaneously. This desirable effect is not achieved without cost and/or introduction of disadvantages, to obtain it, it is necessary to provide as many different constructions of frequency divider units as there are octaves in the instrument. It is obviously more expensive to design, manufacture and inventory a plurality of different frequency divider units, each having a dividing ratio different from the others, than it would be to utilize the same frequency divider unit with each of the master oscillators.
U.S. Pat. No. 3,795,754 describes a system for reducing frequency errors in tone generators of the frequency synthesizer type which employs two master oscillators differing in frequency by a semitone and twelve master frequency dividers respectively corresponding to the twelve notes in an octave. The output of the first master oscillator is applied to six of the master frequency dividers respectively having frequency dividing factors to produce every other note along the scale in an octave, and the output of the second master oscillator is applied to the other six master frequency dividers respectively having frequency dividing factors to produce the remainder of the notes in the octave. The frequency dividing factors for the first group of master frequency dividers are the same as for the second group. The frequencies of the twelve tones are further divided by two to produce octaves below the highest octave. Thus, two master oscillators are required, and while the arrangement substantially reduces the frequency errors of the individual notes as compared to the desired frequencies of the equitempered scale, because the tones for the octaves below the highest octave are derived by dividing the outputs of the twelve master frequency dividers, corresponding tones of each octave are phase-locked and thus do not produce the desired chorus effect when two or more tones from separate octaves are sounded simultaneously.
Some high-priced organs use tone generators that employ a separate tunable oscillator for each note of the scale over a range of many octaves, seventy-three or eighty-five oscillators typically being required. It is generally conceded that such instruments produce superior chorus effects as compared to instruments using the above-described frequency synthesizers and dividers, but in addition to being costly, such systems require periodic tuning and adjustment, a time-consuming, painstaking and consequently costly process.